This invention relates to a process for suspension-polymerizing vinyl chloride. More particularly, the invention relates to a process for preventing polymer scale from adhering to the inner surface of a polymerization vessel in which vinyl chloride is suspension-polymerized, by adding beforehand a specific compound to the polymerization system or by applying the specific compound to the inner surface of the polymerization vessel.
On carrying out the suspension polymerization of vinyl chloride, a larger amount of polymer scale often adheres to the inner surface of the polymerization vessel. When the polymer scale adheres to the inner surface of the polymerization vessel, it causes not only the polymerization yield to be decreased, but also causes difficulty in controlling the temperature in the vessel. This is because the scale hinders conduction of heat on the surface of the vessel, and hence the polymerization reaction cannot be advanced as desired. Furthermore, it sometimes happens that the scale is peeled off from the inner surface of the vessel and mixed into the polymer which is to be the product. Thus, the quality of the product is deteriorated. Adhesion of the polymer scale brings about such disadvantages as stated above, and it is necessary to minimize the amount of scale adhering to the vessel when the polymerization is carried out.
Various attempts have been made to prevent the scale from adhering to the vessel. Most of the attempts adopt a method in which a specific compound is applied beforehand to the inner surface of the polymerization vessel. However, this method is not effective in preventing the scale from adhering when the compound is applied in a small amount. On the other hand, if too much of the compound is applied, the compound is often peeled from the surface of the vessel, and mixed in the polymer and deteriorates the quality of the product, or the compound dissolves in the liquid and acts as a polymerization inhibiting agent, thus making it difficult to carry out the polymerization smoothly. These disadvantages ought to be solved to some extent by improving the compound which is applied. However, it is difficult to obtain a compound which sticks firmly to the inner surface of the polymerization vessel and which maintains a lasting effect of preventing the scale from adhering.
The present applicants previously proposed applying in advance a reaction product of furfural and diamine to the inner surface of a polymerization vessel. Although this reaction product had an excellent effect in preventing the scale from adhering, it could not eliminate the disadvantages mentioned above.
It was also proposed to prevent the scale from adhering by adding beforehand a specific compound to the polymerization system. This proposal is, for example, disclosed in Japanese Patent Publication No. 45-30343. In this publication, many compounds are disclosed as being useful as the compound to be added. The compounds are roughly classified into the following groups: (a) compounds containing an azo-group, nitro-group, nitroso-group, azomethine group, or azine-ring, and nitrogen atom-containing organic compounds, such as amino-compounds, (b) sulfur atom-containing organic compounds having a thiocarbonyl-group, thioether-group or thioalcohol-group, (c) polar organic compounds, such as quinone compounds, ketone compounds, aldehyde compounds, alcohol compounds having more than 6 carbon atoms, and carbonic acid compounds having more than 6 carbon atoms. Although the compounds listed in the publication seem to cover a wide range of chemical substances, the compounds are, in fact, nothing but low molecular weight compounds. Moreover, the publication states that polar organic compounds are preferably used which form a mono-molecular film and further that it is desirable to add the polar organic compounds to a medium in the polymerization system and thus prevent the film from transferring to the medium. Judging from the statements relating to a mono-molecular film, it may be concluded that the publication contemplates nothing but the use of low molecular weight compounds.
The present inventors carried out polymerizations according to the teachings of Japanese Patent Publication No. 45-30343, and discovered that, in so far as the disclosed compounds were used as they were, the compounds did not bring about a sufficient effect in the prevention of scale adhesion, but sometimes caused disadvantages such as deteriorated particle size distribution of the resulting polymers, decreased polymerization rate, and coloring of the resulting polymer. Further, the inventors found that some of the compounds listed in the publication were scarcely effective in preventing the scale adhesion. Thus the inventors found that it was not possible to determine whether the compounds containing the specific groups were effective for preventing scale adhesion.
The inventors carried out further studies to find a more effective method for prevention of scale adhesion. As a result, the inventors have found that the adhesion of scale to a polymerization vessel in the suspension polymerization of vinyl chloride can be prevented when a reaction product of furfural, polyamine and phenol is applied to the inner surface of the polymerization vessel, or when the reaction product is added to the polymerization system. Moreover, the inventors have observed that the reaction product neither hinders the polymerization itself, nor deteriorates the final product by becoming mixed therewith.